When Turks queued up in Istanbul to vote in today's mayoral election rerun, they were casting a vote against their president.

Turkey's ruling AK Party conceded defeat today as Istanbul citizens overwhelmingly voted for oppositional candidate, CHP's Ekrem Imamoglu.

The March mayoral elections were declared illegal by President Recep Tayyip, who feared losing his grip on the country when his party's candidate, Binali Yildirim lost out to Imamoglu by a slim margin in the March ballot.

The March election saw Imamoglu win by a slim margin of 13,000. Unofficial counts of Sunday's election show that this gap widened to about 715,000 votes. 

The controversial mayoral election has gripped Turkey, and ensured a healthy turnout in a city where 10 million are eligible to vote. 

Istanbul residents usually head to the seaside during the warmer months, but Property Turkey director Cameron Deggin, who splits his time between Bodrum and Istanbul, said the Aegean resort was empty, as city dwellers either stayed home or left Bodrum ahead of the election.

"Yesterday there was a mass exodus out of Bodrum. Cars with Istanbul number plates formed a huge line of cars leaving Bodrum, and restaurants, bars and beach clubs were empty.

"That's when I realised this time things will change."

An election with consequences

The vote was seen as a test for democracy, and a bellwether for Erdogan's political future.

The atmosphere in the city was electric, with cars honking and strangers shaking hands when the win was announced. Turks gathered around television screens in squares and in cafes.

The victory will have consequences for the president's hold on the country, with internal power struggles between the factions that work under Turkey's presidential system.

Deggin said Imamoglu's win meant positive change. Despite the mayor being conservative in terms of values and ethics; he is modern and proactive, he said. 

The news was also encouraging for Turkey's ailing economy, he said. 

"A healthy democracy underpins a healthy economy. This change is very encouraging. As Turkey's largest city and most important economic hub Istanbul accounts for almost a third of Turkey's GDP. As the bridge between the east and the west, it's crucial that Istanbul maintains democracy."

Deggin said effect on investment will be a positive one, with investors who have held off in the face of recent uncertainty diving back into Istanbul's market. 

"The city's always been a bridge between east and west. Good democratic processes strengthen that bridge.

"We're entering a new era of prosperity."